The 3-vessel-view demonstrate on the left side of the pulmonary artery the tell-tale sign of a vessel going down: the persistent left superior vena cava (PLSVC) which can empty either in the back of the left atrium or in the coronary sinus as in this case.

The coronary sinus (CS) is very distended:

And in a coronal section perpendicular to the 3-vessel view, the connection of the persistent left superior vena cava to the coronary sinus is well demonstrated.